1. Field of the Invention
The present invention relates to an image-forming apparatus, which reduces the number of signal lines by placing a plurality of signals on one signal line and carrying out multiplexing.
2. Description of the Background Art
Image-forming apparatuses, such as digital photo copiers, facsimile machines, laser printers and the like, are equipped with numerous detachable units, and these units are provided with sensors as detection means for detecting their respective operating statuses. The detection signals from these sensors are supplied to control means (CPUs and so forth) of an image-forming apparatus, and the signal lines provided for this purpose are quite numerous.
Now then, with image-forming apparatuses being equipped with color capabilities, higher performance and greater functionality, the number of these sensor signal lines has shown a tendency to grow. Further, in addition to detection result signals (data signals), power supplies are also needed to make use of these detection means. Inputting the respective detection signals from this large number of detection means into a CPU or other such image-forming control means requires a large number of signal lines and power lines, and image-forming control means are increasing in size. Further, because image-forming control means are installed in locations that are apart from these units and respective types of detection means, the large number of signal lines, as well as the fact that these signal lines wrap all around inside an apparatus have become big obstacles to making such apparatuses simpler, smaller and less costly.
Accordingly, a number of proposals have been put forward in the past for solving these problems. One such proposal, for example, is Japanese Patent Laid-open No. 2002-258691, in which there is proposed an image-forming apparatus, which provides a detachable unit with an I/O expander connected by a serial bus, and which has control means for identifying the type of a detachable unit by the status of the input port of this I/O expander. In this image-forming apparatus, the number of signal lines connecting a unit with the apparatus main body is reduced by identifying the type of unit in accordance with the status of the input port of the I/O expander.
However, according to this past proposal for an image-forming apparatus, the signal lines for each detachable unit comprise a power line, data line, clock line, and ground line, and when viewed in terms of the apparatus as a whole, signal line reduction is still insufficient. Another problem is that when the system configuration (number of input/output means) changes, suitable control means must be provided, leading to higher costs.